The present invention is concerned with transform coefficient coding such as transform coefficients of a transform coefficient block of a picture.
In block-based image and/or video codecs, a picture or frame is coded in units of blocks. Among same, transform-based codecs subject blocks of the picture or frame to a transformation so as to obtain transform coefficient blocks. For example, the picture or frame may be predictively coded with a prediction residual being transform coded in units of blocks and then coding the resulting transform coefficient levels of the transform coefficients of these transform blocks using entropy coding.
In order to increase the efficiency of entropy coding, contexts are used in order to precisely estimate the probability of the symbols of the transform coefficient levels to be coded. However, in the recent years, the demands imposed onto picture and/or image codecs has increased. In addition to the luma and chroma components, codecs sometimes have to convey depth maps, transparity values and so forth. Moreover, the transform block sizes are variable within an increasingly large interval. Due to these varieties, codecs have an increasing number of different contexts with different functions for determining the context from already coded transform coefficients.
A different possibility of achieving high compression rates at a more moderate complexity, is adjusting a symbolization scheme to the coefficients' statistics as precise as possible. However, in order to perform this adaptation closely to the actual statistics, it is also mandatory to take various factors into account thereby necessitating a huge amount of differing symbolization schemes.
Accordingly, there is a need for keeping the complexity of transform coefficient coding low while nevertheless maintaining the possibility of achieving a high coding efficiency.